Fields of Disclosure
The disclosure relates generally to the field of fluid separation. More specifically, the disclosure relates to a method and system for separating fluids in a distillation tower, such as but not limited to during start-up of a distillation tower.
Description of Related Art
This section is intended to introduce various aspects of the art, which may be associated with the present disclosure. This discussion is intended to provide a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
The production of natural gas hydrocarbons, such as methane and ethane, from a reservoir oftentimes carries with it the incidental production of non-hydrocarbon gases. Such gases include contaminants, such as at least one of carbon dioxide (“CO2”), hydrogen sulfide (“H2S”), carbonyl sulfide, carbon disulfide, and various mercaptans. When a feed stream being produced from a reservoir includes these contaminants mixed with hydrocarbons, the stream is oftentimes referred to as “sour gas.”
Many natural gas reservoirs have relatively low percentages of hydrocarbons and relatively high percentages of contaminants. Contaminants may act as a diluent and lower the heat content of hydrocarbons. Additionally, in the presence of water some contaminants can become quite corrosive.
It is desirable to remove contaminants from a stream containing hydrocarbons to produce sweet and concentrated hydrocarbons. Specifications for pipeline quality natural gas typically call for a maximum of 2-4% CO2 and ¼ grain H2S per 100 standard cubic feet (scf) (4 parts per million volume (ppmv)) or 5 milligrams per normal meter cubed (mg/Nm3) H2S. Specifications for lower temperature processes such as natural gas liquefaction plants or nitrogen rejection units typically require less than 50 parts per million (ppm) CO2.
Separating contaminants from hydrocarbons is difficult. Consequently, significant work has been applied to the development of hydrocarbon/contaminant separation methods. These methods can be placed into three general classes: absorption by solvents (physical, chemical, and hybrids), adsorption by solids, and distillation.
Separation by distillation of some mixtures can be relatively simple and, as such, is widely used in the natural gas industry. However, distillation of mixtures of natural gas hydrocarbons, primarily methane, and one of the most common contaminants in natural gas, carbon dioxide, can present significant difficulties. Conventional distillation principles and conventional distillation equipment are predicated on the presence of only vapor and liquid phases throughout the distillation tower. The separation of CO2 from methane by distillation involves temperature and pressure conditions that result in solidification of CO2 if a pipeline or better quality hydrocarbon product is desired. The required temperatures are cold temperatures typically referred to as cryogenic temperatures.
Certain cryogenic distillations can overcome the above mentioned difficulties. These cryogenic distillations provide the appropriate mechanism to handle the formation and subsequent melting of solids during the separation of solid-forming contaminants from hydrocarbons. The formation of solid contaminants in equilibrium with vapor-liquid mixtures of hydrocarbons and contaminants at particular conditions of temperature and pressure takes place in a controlled freeze zone section of a cryogenic distillation tower.
During normal operation, the cryogenic distillation tower operates at steady-state temperature conditions. During normal operation, the operating temperature of the top two sections that may be in a distillation tower, e.g., the controlled freeze zone section, and the upper section, are cold. As a result of the cold operating temperatures, the concentration of CO2 within the upper section and the controlled freeze zone section of the distillation tower is low; the CO2 is knocked down by cold liquid sprays from a spray assembly within the controlled freeze zone section.
During abnormal operations, the cryogenic distillation tower does not operate at steady-state temperature conditions. The operating temperatures of the three main sections that may be in a distillation tower are not cold. As a result, the warmer operating temperatures prevent sufficient CO2 from being knocked out, thereby leading to a higher concentration of CO2 within the upper section and the controlled freeze zone section of the distillation tower than would be present during normal operation. The increased CO2 concentration may result in CO2 solidifying outside of the middle controlled freeze zone section. The challenge during an abnormal operation is to get from an abnormal operation to a normal operation. Examples of when abnormal operations may occur include but are not limited to during start-up of the distillation tower. During start-up, the distillation tower may be at or near-ambient temperatures.
A need exists for improved technology that can better facilitate going from an abnormal operation to a normal operation so as to separate fluids in a distillation tower, such as but not limited to during start-up of a distillation tower.